1. Field of the Invention
The present invention relates to an austenite stainless steel plate with excellent surface brightness and corrosion resistance.
2. Description of the Prior Art
Because austenite stainless steel plate represented by SUS 304 has excellent thermal resistance, corrosion resistance, processability and the like, the steel plate has been put to a variety of uses with attention focused on the characteristic properties thereof. For producing a cold-rolled steel strip from the austenite stainless steel slab, generally, annealing is carried out after cold rolling at the finish process, to prepare the steel strip with specified properties, depending on the use.
The annealing process at the finish process is carried out in strongly reducing atmosphere or in combustive gas atmosphere. The former process is called bright annealing (BA), and the oxidized film (scale) formed on the steel strip surface by such annealing is so extremely thin that the surface brightness remaining on the material immediately after rolling out can be retained practically as it is. Because H.sub.2 and N.sub.2 gases in mixture are used in the atmosphere, however, the equipment therefor gets complex, which disadvantageously makes such annealing costly. On the other side, by the latter process, a relatively thick scale layer is generated during annealing, and therefore, the resulting steel plate at such state adversely affects the corrosion resistance and the die life during forming. Thus, pickling is indispensable for descaling in case that annealing is carried out in the combustive gas atmosphere by the latter process. Consequently, the surface brightness of the finished plate is poorer than the rolled-out surface.
As to the pickling, a great number of research works have been conducted conventionally, with a lot of technical references in publication. For example, Japanese Patent Publication No. Sho 38-12162, Japanese Patent Laid-open No. Sho 59-59900 or Stainless Steel Guide Book (edited by Masayoshi Hasegawa, ed., published by Nikkan Industrial Newspaper, 1973, p.839) discloses a so-called salt process comprising immersing the annealed steel strip in an alkali molten salt or a method comprising an electrolytic process of the annealed steel strip in a neutral salt solution and a subsequent immersion process of the resulting strip in an acid solution such as sulfuric acid, nitric acid, and nitrate salt or a subsequent electrolytic process of the strip.
At the pickling process for the descaling of austenite stainless steel strip, conventionally,an acid mixture comprising nitric acid and hydrofluoric acid has been used generally. When the acid mixture is used, however,the surface of the steel strip at a state such that not only the crystal intergranular phase but also the crystal intragranular phase is eroded by the acid mixture. Subjecting the steel strip at such state to "skinpass rolling" under a mild pressure, the surface brightness is severely deteriorated, compared with the surface brightness generated by conventional rolling. So as to produce a steel strip with sufficient surface brightness, accordingly, a polishing process has been indispensable after pickling. For the purpose of reducing the burdens of works during the polishing process, conventionally, several proposals have been made as described below.
For example, Japanese Patent Publication No. Sho 62-60164 proposes a technique comprising after cold rolling austenite stainless steel strip, polishing the surface by means of a cloth belt prior to annealing and pickling, and subjecting the resulting steel plate, if necessary, to "skinpass rolling" to prepare a product. However, the technique requires large-scale equipment for surface polishing by means of a cloth belt, which disadvantageously escalates the production cost enormously.
For the purpose of improving the polishing properties of the steel strip surface, for example, Japanese Patent Publication No. Hei 3-60920 proposes to reduce the intergranular eroding by descaling of a hot-rolled annealed steel plate in an acid mixture of given concentrations of nitric acid and hydrofluoric acid. By the technique, however, the surface of the steel plate is so severely solubilized above the necessary level that superficial non-uniformity and irregularity readily develop; and the surface brightness after pickling is not necessarily good. Thus, no improvement is realized as to the requirement of the polishing work.
Additionally, Japanese Patent Laid-open No. Hei 6-280064 discloses a method for improving the polishing properties, by skipping annealing after hot rolling but carrying out annealing and pickling within specific ranges at the finish process to prepare the depth of the pickling-generated micro-groove at 1.0 .mu.m or less. Even by the method, however, the polishing properties are more or less improved, but the surface brightness of the steel strip after pickling is deteriorated. So as to improve the surface brightness, additional "skinpass rolling" should essentially be enforced.
Still furthermore, Japanese Patent Laid-open No. Hei 6-17271 discloses a technique to improve the polishing properties, comprising of suppressing the depth of the intergranular groove as much as possible by defining the annealing conditions and the concentrations of nitric acid and hydrofluoric acid, thereby preparing the depth of the intergranular groove below 1.0 .mu.m or less. The method has another problem however in that scale still remains on the surface after pickling and the corrosion resistance is severely deteriorated unless the scale is removed by polishing and the like.
Further, Japanese Patent Laid-open No. Hei 7-113187 discloses a method for promoting the whitening of the surface of a steel strip through pickling in sulfuric acid at a specific concentration, in place of acid mixture composed of nitric acid and hydrofluoric acid. Even by the technique, however, polishing is inevitable for procuring sufficient surface brightness after skinpass rolling, by the definition of the acid concentration, the surface chromium-depleted layer cannot sufficiently be removed, which is disadvantageous because the corrosion resistance after pickling is poorer than the resistance brought about by conventional methods.
From the respect of the improvement of corrosion resistance, still yet furthermore, Japanese Patent Laid-open No. Hei 6-88297 or Japanese Patent Laid-open No. Hei 6-88300 discloses a method for removing the Si-concentrated layer on the steel strip surface, by limiting the pH of a solution for neutral salt electrolysis prior to pickling in an acid mixture or by limiting the electrolytic conditions for nitrate electrolysis after the pickling. Under the limitation of the pH of the solution for neutral salt electrolysis and of the conditions for nitrate electrolysis, the surface is highly solubilized and the intergranular phase in particular can be readily eroded. The corrosion resistance is improved, indeed, but on the contrary, no sufficient surface brightness can be realized, disadvantageously.
As has been described above, all the known techniques described insofar have problems in that polishing is essential after pickling for procuring satisfactory surface brightness in austenite stainless steel and additionally in that the corrosion resistance of the resulting steel strip is unsatisfactory.